Hybrid vehicles, such as plug-in hybrid vehicles, may have two modes of operation: an engine-off mode and an engine-on mode. While in the engine-off mode, power to operate the vehicle may be supplied by stored electrical energy. While in the engine-on mode, the vehicle may operate using engine power. By switching between electrical and engine power sources, engine operation times may be reduced, thereby reducing overall carbon emissions from the vehicle. However, shorter engine operation times may lead to insufficient purging of fuel vapors from the vehicle's emission control system. Additionally, refueling and emission control system leak detection operations that are dependent on pressures and vacuums generated during engine operation may also be affected by the shorter engine operation times in hybrid vehicles.
Various strategies have been developed to address fuel vapor control and management in hybrid vehicle systems. Example approaches include separating storage of refueling vapors from storage of diurnal vapors by adding a fuel tank isolation valve (FTIV) between a fuel tank and a fuel vapor retaining canister, and allowing refueling vapor purging to the canister during refueling events, and engine-on purging methods. The separation of diurnal and refueling vapors allows a pressure to be generated in the fuel tank, while application of alternative vacuum sources allows a vacuum to be generated in the canister.
One example approach for fuel vapor management is shown by Ito el al. in U.S. Pat. No. 6,557,401. Therein, leak detection is performed in two stages. First a fuel tank is sealed and a change in fuel tank pressure is measured over time. Next, a vacuum is applied to a canister and the presence of leaks is determined based on changes in the fuel tank pressure and canister pressure over time.
Another example approach is shown by Takagi et al. in U.S. Pat. No. 6,761,154. Therein, leak detection is performed by operating a pump to apply a vacuum on the canister, followed by monitoring a change in the canister pressure over time. A valve disposed between the fuel tank and the canister is then opened to apply the vacuum to the fuel tank, followed by monitoring a change in fuel tank pressure over time. Presence of leaks may be determined based on changes in first the carbon canister pressure over time and then the fuel tank pressure over time.
However, the inventors herein have recognized potential issues with these approaches. As one example, these approaches fail to address the transitory nature of pressure and vacuum accumulation in a hybrid vehicle system due to infrequent and irregular engine operation. For example, the shorter duration of engine operation in hybrid vehicles may lead to lower amounts of vacuum being generated during an engine-on mode, such that insufficient vacuum may be present in the fuel tank during the leak detection. As a result, there may not be sufficient pressure and/or vacuum for detecting leaks in both the fuel tank and the carbon canister. Since leak detection in the above approaches fuel tank is tied to leak detection in the carbon canister, insufficient pressure and/or vacuum may lead to incomplete leak detection. Operation of a dedicated pump to generate the required vacuum may increase system cost and power consumption.
In one example, some of the above issues may be addressed by a method of monitoring a vehicle fuel vapor recovery system including a fuel tank, a canister, and a vacuum accumulator, the vacuum accumulator including a venturi. The method may comprise, during a first engine-on condition, flowing air and/or exhaust through the venturi to generate a vacuum, and storing the generated vacuum in the vacuum accumulator; and during a subsequent engine-off condition, applying vacuum from the vacuum accumulator on the canister, and indicating fuel vapor recovery system degradation based on a change in fuel vapor recovery system pressure following vacuum application.
In one example, a fuel vapor recovery system for a hybrid vehicle may comprise a fuel tank coupled to fuel vapor retaining device (such as a carbon canister) via a fuel tank isolation valve (FTIV). The canister may be coupled to the engine intake via a canister purge valve (CPV). The canister may be further coupled to a vacuum accumulator via a vacuum accumulator valve (VAV). As such, the FTIV may be maintained in a closed state during vehicle operation and may be selectively opened during refueling and diurnal vapor purging conditions. By maintaining the FTIV closed, the fuel vapor circuit may be divided into a canister side and a fuel tank side. Refueling vapors may be retained in the canister on the canister side of the circuit while diurnal vapors may be retained in the fuel tank on the fuel tank side of the circuit.
A first pressure sensor may be coupled to the fuel tank to estimate a pressure of the fuel tank side of the circuit, while a second pressure sensor may be coupled to the carbon canister to estimate a pressure of the canister side of the circuit. Based on input from various sensors, such as the pressure sensors, and further based on vehicle operating conditions, a controller may adjust various actuators, such as the VAV, the CPV, the FTIV, and a canister vent valve (CVV), to enable fuel tank refueling, purging of stored fuel vapors, and leak detection in the fuel vapor recovery system.
In one example, the vacuum accumulator may be coupled to a venturi disposed in an air flow path such that a vacuum may be accumulated therein independent of the vehicle engine operation mode. For example, the venturi may be mounted on the underside of a vehicle body so that it receives ambient air flow from vehicle motion in either of the engine-off or engine-on modes of operation and stores vacuum accordingly. In another example, the venturi may be disposed in the exhaust pathway of a brake booster pump so that it accumulates vacuum during brake operation in either of the engine-on or engine-off modes of operation. In this manner, vacuum may be stored in the vacuum accumulator, irrespective of engine operation mode, for later use, for example during a later leak detection routine. By storing vacuum and applying the stored vacuum at a later time, the reliance on an engine-on operation mode of the hybrid vehicle and/or a dedicated vacuum pump may be reduced.
Further still, during leak detection, an order of detecting leaks in the components of the fuel vapor recovery system may be adjusted based on an amount of vacuum available for the leak detection. For example, if sufficient engine-off natural vacuum is not available, vacuum from the vacuum accumulator may be applied by opening the VAV. Herein, first the carbon canister may be checked for leaks, then the operation of the FTIV may be verified, and then the fuel tank may be tested for leaks. In comparison, if sufficient engine-off natural vacuum is available for leak detection, the fuel tank may be tested for leaks first, then the operation of the FTIV may be determined, and finally the carbon canister may be checked for leaks.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.